This patch implements a .from_buffer(source, offset=0) class method from
ctypes types.  'source' must expose a writeable buffer interface; the
created ctypes instance will share the internal buffer of the source
object; also it holds a reference to the source object to keep it alive.

The usual problems with the buffer interface are not hidden.

Open question:
Should there be an additional 'copy_from_buffer' class method that
accepts read-only buffer interface (as an alternative, an optional
'copy=False' parameter could be introduced)?

The suggestion by Lenard Lindstrom was an additional method named
'from_buffer_copy'.

Implemented from_buffer() and from_buffer_copy() as rev 62497 to SVN trunk.